Weeds cost farmers billions of dollars annually in crop losses and in the expense of keeping the weeds under control. Much of the cost of intertillage of row crops, maintenance of fallow, seedbed preparation, and seed cleaning is chargeable to weed control. Another expensive item is suppression of weeds along highways and railroad right-of-ways, and in irrigation ditches, navigation channels, yards, parks, grounds, and home gardens. Ragweed pollen is the source of annual periodic distress to several million hay fever sufferers. Poison ivy, poison oak, poison sumac, nettles, thistles, sandburs, and puncturevine also bring pain to millions. The barberry bush, which spreads the black-stem rust of grains and grasses, can be regarded as a weed. Weeds also serve as hosts for other crop diseases as well as for insect pests.
The losses caused by weeds in agricultural production environments include decrease in crop yield, reduced crop quality, increased irrigation costs, increased harvesting costs, decreased land value, injury to livestock, and crop damage from insects and diseases harbored by the weeds.
Chemical herbicides have provided an effective method of weed control in the past. However, the public has become concerned about the amount of chemicals applied to the food that they consume, to the land on which they live, and to the ground water which they use. Stringent restrictions on the use and development of new herbicides and the elimination of some effective herbicides from the market place have limited economical and effective means for controlling costly weed problems.
A problem has been identified after years of use of chemical herbicides on commercial agricultural land, i.e., the lack of control of certain weeds has allowed these weeds to take over the areas where, without the use of chemical herbicides, they were excluded by more hardy weeds. Removal of the more competitive weeds with chemical herbicides has left an ecological void that has been filled by the less competitive weeds that are resistant to the herbicides. Weeds that were of minor importance at one time have spread rapidly throughout the areas where they are found and are now considered major weed problems. In addition to the inadequacy of control of all weeds, chemicals also can damage the crop plants, sometimes injure nontarget organisms in the environment, and can leave undesirable residues in water and harvested products and carry-over in subsequent crops.
Microbial herbicides are plant pathogens which are effective, when used according to the process disclosed herein, in controlling weeds or other undesirable vegetation without adversely affecting the growth and yield of the desired field crop. The composition of a microbial herbicide includes spores or cells of the plant pathogen or any portion of the organism that is capable of infecting the weed. The use of microbial herbicides is becoming an increasingly important alternative to chemical herbicides. This importance is accompanied by the issuance of several patents for microbial herbicides and their use. Some of these patents, by way of illustration, are as follows: U.S. Pat. No. 3,849,104 (control of northern jointvech with Colletotrichum gloeosporioides Penz. aeschynomene); U.S. Pat. No. 3,999,973 (control of prickly sida [teaweed] and other weeds with Colletotrichum malvarum); U.S. Pat. No. 4,162,912 (control of milkweed vine with Araujia mosaic virus); U.S. Pat. No. 4,263,036 (control of Hydrilla verticillata with Fusarium roseum Culmorum); U.S. Pat. No. 4,390,360 (control of sicklepod, showy crotalaria, and coffee senna with Alternaria cassiae); and U.S. Pat. No. 4,419,129 (control of prickly sida, velvetleaf, and spurred anoda with fungal pathogens).
Microbial herbicides have been developed specifically for control of weeds which are not adequately controlled by chemcial herbicides. Examples include Colletotrichm gloeosporioides f. sp. aeschynomene for control of northern jointvetch in rice; Alternaria cassiae for control of sicklepod in soybeans, cotton, and peanuts; and Fusarium lateritium for control of velvetleaf in soybeans. In each of these cases the weed is not effectively controlled by the chemical herbicides currently labeled for use in the respective cropping system. The factors currently limiting in commercialization of microbial herbicides are the high cost of production, limited spectrum of weed control, and the narrow range of environmental conditions in which these pathogens will infect the host.
The effects of herbicides on plant diseases was recently reviewed by Altman (Altman, J. and Campbell, L. C. [1977] Ann. Rev. Phytophatol. 15:373-375). Altman reported that herbicides may either increase or reduce plant disease and severity. There are five major herbicide effects which may lead to increased disease: (a) a reduction in the biochemical defenses of the host against the pathogen; (b) reduction of structural defenses of the host; (c) stimulation of increased exudation from host plants; (d) stimulation of pathogen growth and/or production of chemicals which damage the plant; and (e) inhibition of microflora competing with potential pathogens. There are four major effects of herbicides which lead to decreased disease incidence and/or severity: (a) increased host biochemical defenses; (b) increased host structural defenses; (c) stimulation of microflora competing with potential pathogens; and (d) a decrease in either the pathogen's growth or its production of chemicals which are damaging to plants. At the current state of chemical herbicide and microbial herbicide art, there is no method of predicting the interaction (neutral, antagonistic, or synergistic) between a microbial herbicide and a chemical herbicide in controlling a specific weed or unwanted vegetation.
Prior art in the area of microbial herbicide and chemical herbicide interactions indicates that foliar application of mixtures of a microbial herbicide and a chemical herbicide results in antagonism and reduced efficacy of the microbial herbicide. Plant pathogens can break down chemical herbicides and chemical herbicides can be fungicidal (Wilson, C. L. [1969] Ann. Rev. Phytopathol. 7:424). Examples of positive interactions between microbial herbicides and chemical herbicides require that the microbial herbicide be applied either before or after the application of the chemical herbicide (Klerk, R. A., Smith, Jr., R. J. and TeBeest, D. O. [1985] Weed Science 33:95-99). Multiple applications of pest control products is expensive and commercially undesirable. The commercially viable methods for the application of a combination product (such as a microbial herbicide and a chemical herbicide) are a "tank mix," and a "package mix." Tank mixing is a process by which two or more components of a pest control program are added to the same spray tank and this mixture is applied to the field. The components may be packaged together (package mix) or separately (tank mix) but the components must be compatible when added to the spray tank. Mixtures are applied to the field with one application. Applying a mixture reduces fuel consumption, machinery wear, and operator time; and preserves the soil texture by reducing soil compaction. At this stage in the herbicide art there is no known way to predict success, if any, in combining a chemical herbicide with a microbial herbicide.
We have discovered that mixtures of microbial herbicides and chemical herbicides, and some chemical plant growth regulators, are synergistic in their activity when applied to the foliage of the host weed of the microbial herbicide. This is the first report of synergy between microbial herbicides and chemical herbicides applied as mixtures. This synergy will greatly increase the value of microbial herbicides by reducing the amount of microbial herbicide applied, reducing the environmental limitations of the microbial herbicide, and increasing the spectrum of weed control of some herbicide treatments.